It's the modulus operator, which finds the remainder of a number when divided out into the second number.

So let's say you start with this: 7%2. To determine its value, you take 7, and subtract 2 until you cannot anymore. Like so:
7 - 2 = 5
5 - 2 = 3
3 - 2 = 1
(Can't subtract 2 anymore, since 1 - 2 < 0)
Therefore, 7%2 == 1. You'll also notice if you do long division for 7 divided by 2, the remainder will be 1. This holds true for any modulus operation.

In your case, basically it's checking to see if "i" is divisible by 5. That meas 5, 10, 15, ..... etc.